45.6.8.2 Baud Rate

In SPI mode, the baud rate generator operates in the same way as in USART Synchronous mode. See “Baud Rate in Synchronous Mode or SPI Mode”. However, there are some restrictions:

In SPI Host mode:

• The external clock SCK must not be selected (USCLKS ≠ 0x3), and US_MR.CLKO must be written to ‘1’, in order to generate correctly the serial clock on the SCK pin.
• To obtain correct behavior of the receiver and the transmitter, the value programmed in US_BRGR.CD must be greater than or equal to 6.
• If the divided peripheral clock is selected, the value programmed in CD must be even to ensure a 50:50 mark/space ratio on the SCK pin. This value can be odd if the peripheral clock is selected.

In SPI Client mode:

• The external clock (SCK) selection is forced regardless of the value of the US_MR.USCLKS. Likewise, the value written in US_BRGR has no effect, because the clock is provided directly by the signal on the USART SCK pin.
• To obtain correct behavior of the receiver and the transmitter, the external clock (SCK) frequency must be at least 6 times lower than the system clock.